1. Field of the Invention
The present invention relates to a circuit substrate provided with a plurality of heat generating elements and a liquid discharging apparatus and, in particular, to a circuit substrate used for a liquid discharging apparatus in which a heat generating element converts an electric energy into a thermal energy and the heat energy is used to emit a liquid.
2. Description of the Related Art
A conventional circuit substrate is described below with an inkjet head as an example.
An inkjet recording apparatus emits ink as a minute droplet from an orifice for discharging to a recording member to record an image thereon. Theoretically, a heat generating element converts an electric energy into a heat energy and the heat energy generates a bubble in the ink. The action of the bubble causes an orifice for discharging at the tip of a liquid discharging head to emit a droplet to stick to the recording member to record an image thereon. For this reason, such a liquid discharging head has a circuit substrate including a plurality of heat generating elements for converting an electric energy into a heat energy. Specifically, as illustrated in FIG. 5, a diffusion region 301 being a source and a drain region is formed on a silicon (Si) substrate 30 and a gate electrode 302 is arranged through an insulating film, forming a transistor portion 31 being a power transistor. A first wiring layer 32 is formed on the Si substrate 30 through an insulating layer and connected to the diffusion region 301 being a source and a drain region. A third wiring layer 36 forms a pair of electrodes connected to a resistor 35. One of the pair of electrodes is connected to the first wiring layer 32 connected to the source and the drain region through a second wiring layer 34. The resistor 35 between the pair of electrodes forms a heat generating portion. The pair of electrodes and the heat generating portion of the resistor 35 form the heat generating element. The second wiring layer 34 is provided between the first and the third wiring layer 32 and 36. The first wiring layer 32 is electrically connected to the third wiring layer 36.
There are formed a protective layer (passivation) 37 for protecting the third wiring layer and the resistor 35 from the ink, a cavitation resistance film 38 for protecting the protective layer from chemical or physical damages caused by heating and an interlayer film 33.
The circuit substrate used for the liquid discharging apparatus has a plurality of the aforementioned heating generating elements with a high density to record an image. Each heating generating element is connected in series with a power transistor (the transistor portion 31 in FIG. 5) for turning on and off current flowing through the heating generating element. In addition, an orifice for discharging is formed over the circuit substrate thereby providing a liquid discharging apparatus.
In recent years, there has been demanded to reduce a pitch between the heating generating elements and to print images with a small droplet and a high density. This has demanded to miniaturize a driving circuit including a heating generating element and a power transistor. The number of wirings formed over the heating generating element needs to be increased and wiring layers need to be provided under the heating generating element.
In a case where a density among the elements is 1200 dpi in terms of realizing a high density printing, the wirings are three-layered in all. The first wiring layer uses AlSi, for example, to be connected to the diffusion region of the semiconductor substrate. The second and the third wiring layer are power source wirings for driving the heating generating elements. The power source wiring thorough which a large current flows uses a highly reliable AlCu, for example. The third wiring layer forms a pair of electrodes of the heating generating element. A relevant configuration is described in Japanese Patent Application Laid-Open No. 2002-313942.
However, for the above structure, heat from the heating generating element causes a phenomenon in which Si in the first wiring layer of AlSi makes a solid solute diffusion to the second wiring layer of AlCu. For this reason, the Si erodes Si in the Si substrate and penetrates the diffusion region (illustrated by a “penetrating through portion” in FIG. 5), which may cause a problem that leakage into the substrate occurs. The diffusion of Si to the second wiring layer causes segregation and hillock of Si, produces a crack illustrated in FIG. 5 and may cause a problem that the ink durability of the heating generating element is degraded.
The object of the present invention is to emit a droplet with high density by reducing width between the heating generating elements in the circuit substrate provided with a plurality of the heating generating elements and improve the reliability of the heating generating element.